The present invention relates generally to fault detection in a polyphase alternating current (a.c.) to direct current (d.c.) conversion system and more particularly to the detection of an improperly conducting controlled rectifier within a bridge circuit utilized to supply a d.c. motor with power from an a.c. source and to means for correcting or rendering nonconductive such an improperly conducting rectifier.
It is well known in the art to provide a system for the conversion of a.c. to d.c. through the use of a full wave rectification bridge employing a plurality of controlled rectifiers which, today, are usually thyristors of the type known as silicon controlled rectifiers (SCRs). It is also well known that, whether the system be single phase or polyphase, the value of the d.c. voltage at the output of the rectifying bridge can be varied by controlling the phase firing angle of the individual bridge rectifiers. Phase angle control, as is well understood in the art, means that the individual rectifiers are fired at controllable times within the applied voltage cycle such that they are conductive only for a portion of the cycle. The use of such bridge circuits for the control of d.c. motors is also well known in the art and such systems normally employ a variety of feedback and control circuits to adjust the phase firing angle of the rectifier bridge so as to control the motor operation through the control of the voltage applied to the motor. An example of such a system employing a current limit feature is shown and described in U.S. Pat. No. 3,526,819, "Current Limit For Motor Control Systems" by C. E. Graf, issued Sept. 1, 1970, which patent is specifically incorporated hereinto by reference.
Controlled rectifier power conversion systems for motor control such as are here being described are particularly susceptible to what are known as d.c. faults or "shoot-throughs" when the motor is operating in the regenerative mode. A d.c. fault occurs when one or more of the rectifiers of the bridge are conducting at a time when the should be nonconducting. These faults or shoot-throughs may result from a variety of reasons including the failure of a particular rectifier to fire or conduct at its appointed time because of an intermittently faulty rectifier firing circuit connector or firing control. It may also be the result of the failure of one rectifier to commutate off its predecessor due to excessive current or lack of sufficient volt-seconds due to a.c. supplied line voltage reduction. Other possible causes include the false firing of one rectifier in the same a.c. supply phase as the rectifier properly fired due to excessive anode to cathode change of voltage with respect to time and electrical noise coupled into the firing circuit associated with the rectifiers. Regardless of the cause of the d.c. fault or shoot-through, the end result may be fuse failure and sudden shut down of the drive or, in a more exaggerated case, the excessive currents may cause damage to the motor, if the motor is not otherwise protected.